netdev - Re: ipv6 fragmentation-related panic in netfilter

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Message-ID: <20131101084529.GL31491@secunet.com>
Date:	Fri, 1 Nov 2013 09:45:29 +0100
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Tomas Hlavacek <tmshlvck@...il.com>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: ipv6 fragmentation-related panic in netfilter

On Wed, Oct 30, 2013 at 12:07:11AM +0000, Patrick McHardy wrote:
> 
> The problem is that the reassembled packet is referenced by the individual
> fragments, so we trigger the BUG_ON in pskb_expand_head(). In this
> particular case the case we BUG() on is actually OK, but I'm looking at
> a way we can fix this without special casing. Hope to have a patch for
> testing in the next hours.

Just for the record. I'm observing similar, quite reproducable crashes when
receiving fragmented icmp echo request packets on an IPsec gateway with
nf_conntrack_ipv6.

Since git commit 58a317f10 ("netfilter: ipv6: add IPv6 NAT support")
netfilter might insert a reassembled ipv6 packet with a shared skb and
local_df = 1 to the ok function. In case of xfrm, __xfrm6_output()
fragments the packet again and when adjusting the headroom later, we
crash because of a shared skb.

I can fix it by checking for a shared skb in ip6_fragment() and do
slow path fragmentation then. But we never needed such a check in
ip6_fragment(), so it's maybe better to fix it in netfilter.
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